The present invention is concerned with an improved X-ray lithography system which is adapted to replicate submicron mask patterns by proximity focusing soft X-rays on a photosensitive material to produce microelectronic circuits, and is more particularly concerned with such a system which is capable of producing finer scale patterns than are possible with the visible light lithographic processes now widely used in the semiconductor industry, and which is capable of achieving higher LSI circuit functional densities than have been produced by X-ray lithography systems suggested heretofore, by reducing the effects of geometric distortion and nonuniform illumination. These improved results are achieved by employing in the system a grazing incidence optical system which produces substantially collimated X-rays that eliminate geometric distortions of the pattern that would result if uncollimated beams were employed. The device of the present invention provides advantages additional to collimation, e.g., the ability to illuminate large patterns several centimeters square, and to do so with good uniformity and high efficiency. The system has applications to X-ray shadow microscopy as well as lithography.
The method currently employed in the industry for the fabrication of electronic and optical microdevices by microfabrication techniques, wherein detailed circuit patterns on a mask are irradiated onto a silicon wafer that is covered with a photosensitive material, is to employ, as the irradiating source, visible/ultraviolet light (on the order of 4,000 A wavelength). Two approaches are normally used in such known lithography systems, i.e., proximity printing and projection printing. The minimum line widths achieved in commercial systems are about 2.5 to 3.5 micrometers, although finer lines, e.g., about 0.7 micrometers in width, have been achieved in laboratory arrangements. As integrated circuit technology advances, there is a need for higher-density IC's, more complex structures with more elements, finer line widths and smaller feature spacing. One attempting to meet these requirements with visible light lithography encounters fundamental limitations resulting from diffraction, and also encounters substantially increased costs.
X-ray lithography, wherein soft X-rays are substituted for visible or ultraviolet radiation, has been suggested as a submicrometer lithographic technique that is potentially high in throughput and relatively inexpensive. Such X-ray lithography systems constitute, at the present time, a laboratory development. X-rays of 4-50 A wavelength are used to print replicas of gold mesh patterns, or electron-beam generated masks, on photosensitive materials. Various illumination schemes have been employed in these laboratory X-ray lithography systems.
To achieve best results in an X-ray lithography system, it is desirable to employ a collimated beam of X-rays to avoid geometrical spreading and distortion effects which would occur if rays were to pass through the mask at nonnormal angles and follow slanted paths to the photosensitive material on the wafer. The most common approach used heretofore in an effort to achieve such collimation has been to place a spatially small, fairly conventional (i.e., electron beam/anode) source of X-rays at a comparatively large distance from the mask/wafer, the source distance being made sufficiently large (e.g., 50 l centimeters) to achieve partial collimation. In such arrangements, the mask-wafer separation is made small (e.g., 40 micrometers). Lines as narrow as 0.16 micrometer have been achieved but, more typically, lines of 1 micrometer width are produced over a wafer of several centimeters in dimension.
Systems of the type described above tend to be relatively inefficient since good collimation requires a comparatively large source-to-mask distance. Since the flux incident on the mask falls off as the inverse square of the source-to-mask distance, only a very small fraction of the source output is actually used.
Other types of system have been suggested in an effort to avoid geometrical spreading and distortion effects. One such alternative system suggested heretofore employs contact focusing, which allows good resolution over several centimeters with an extended (hence high power) source. Such contact focusing systems, employed in laboratories at the present time, will in all probability not be suited to production work because of mask damage.
Another system which has been suggested is to use, in place of a conventional X-ray source, an unconventional source such as a pulsed laser plasma. Such a source has been suggested since it tends to reduce the exposure time by taking advantage of the flux and spectrum available from this type of source. The collimation problem is, however, the same as that for a conventional source, and a parallel beam is obtained only at a substantial distance from the source and at a corresponding penalty in flux.
Another variation which has been suggested heretofore is to employ synchrotron radiation. Such radiation is intense, collimated, and has a favorable continuous spectrum. A major disadvantage of this approach, however, is the comparatively high capital cost of a synchrotron facility; the necessary investment is such that only a small number of synchrotron laboratories exist, and hence a microelectronics manufacturer would be obliged to share a facility with other users, including competing firms. A secondary disadvantage of synchrotron sources is the nonuniform illumination in the beam, which limits the area over which it is possible to print with optimum exposure.
The present invention is intended to obviate these disadvantages of the prior art, and to provide an X-ray lithography system which is relatively inexpensive but which is nevertheless capable of achieving the production of finer scale patterns and higher-density IC's than can be achieved by visible/ultraviolet light and/or X-ray lithography systems used heretofore.